1. Field of the Invention
One embodiment of the present invention relates to a light-emitting element, or a display device, an electronic device, and a lighting device each including the light-emitting element.
Note that one embodiment of the present invention is not limited to the above technical field. The technical field of one embodiment of the invention disclosed in this specification and the like relates to an object, a method, or a manufacturing method. In addition, one embodiment of the present invention relates to a process, a machine, manufacture, or a composition of matter. Specifically, examples of the technical field of one embodiment of the present invention disclosed in this specification include a semiconductor device, a display device, a liquid crystal display device, a light-emitting device, a lighting device, a power storage device, a memory device, a method for driving any of them, and a method for manufacturing any of them.
2. Description of the Related Art
In recent years, research and development have been extensively conducted on light-emitting elements using electroluminescence (EL). In a basic structure of such a light-emitting element, a layer containing a light-emitting substance (an EL layer) is interposed between a pair of electrodes. By applying a voltage between the pair of electrodes of this element, light emission from the light-emitting substance can be obtained.
Since the above light-emitting element is of a self-luminous type, a display device using this light-emitting element has advantages such as high visibility, no necessity of a backlight, low power consumption, and the like. Further, the display device also has advantages in that it can be formed to be thin and lightweight, and has high response speed.
In the case where the above light-emitting element is used for a display device, there are the following two methods: a method of providing subpixels in a pixel with EL layers having functions of emitting light of different colors (hereinafter referred to as a separate coloring method) and a method of providing subpixels in a pixel with, for example, a common EL layer having a function of emitting white light and color filters each having a function of transmitting light of a different color (hereinafter referred to as a white EL+color filter method, and it is noted that the emission color of the common EL layer is not limited to white).
One of the advantages of the white EL+color filter method is that the EL layer can be shared by all of the subpixels. Therefore, compared with the separate coloring method, loss of a material of the EL layer and cost of patterning for forming the EL layer can be reduced; thus, display devices can be manufactured at low cost with high productivity. Further, although it is necessary, in the separate coloring method, to provide a space between the subpixels to prevent mixture of the materials of the EL layers in the subpixels, the white EL+color filter method does not need such a space and therefore enables a high-resolution display device having higher pixel density.
The light-emitting element can emit light of a variety of colors depending on the kind of light-emitting substance included in the EL layer. A light-emitting element which can emit white light or light of color close to white with high efficiency has been particularly demanded for the application to a lighting device and a display device utilizing the white EL+color filter method. In addition, a light-emitting element having low power consumption is required.
In a light-emitting element (e.g., an organic EL element) whose EL layer contains an organic compound as a light-emitting substance and is provided between a pair of electrodes, application of a voltage between the pair of electrodes causes injection of electrons from a cathode and holes from an anode into the EL layer having a light-emitting property and thus a current flows. By recombination of the injected electrons and holes, the organic compound having a light-emitting property is brought into an excited state to provide light emission.
Note that an excited state formed by an organic compound can be a singlet excited state (S*) or a triplet excited state (T*). Light emission from the singlet excited state is referred to as fluorescence, and light emission from the triplet excited state is referred to as phosphorescence. The statistical generation ratio of the excited states in the light-emitting element is S*:T*=1:3. In other words, a light-emitting element containing a compound emitting phosphorescence has higher emission efficiency than a light-emitting element containing a compound emitting fluorescence. Therefore, light-emitting elements including phosphorescent compounds capable of converting a triplet excited state into light emission have been actively developed in recent years. For example, a white-light-emitting element including a plurality of phosphorescent compounds is disclosed in Patent Document 1.
Among light-emitting elements including phosphorescent compounds, in particular, a light-emitting element that emits blue light has not yet been put into practical use because it is difficult to develop a stable compound having a high triplet excited energy level. For this reason, the development of a more stable fluorescent compound for a light-emitting element that emits blue light or a white-light-emitting element in which blue light emission occurs has been conducted and a technique for increasing the emission efficiency of a light-emitting element including a fluorescent compound (fluorescent light-emitting element) has been searched.
As a light emission mechanism capable of converting part of a triplet excited state into light emission, triplet-triplet annihilation (TTA) is known. The term TTA refers to a process in which, when two triplet excitons approach each other, excited energy is transferred and spin angular momentum are exchanged to form a singlet exciton (Non-Patent Document 1).
As one of materials capable of partly converting the triplet excited state into light emission, a thermally activated delayed fluorescence (TADF) substance has been known. In a thermally activated delayed fluorescence substance, a singlet excited state is generated from a triplet excited state by reverse intersystem crossing, and the singlet excited state is converted into light emission.
In order to increase emission efficiency of a light-emitting element using a thermally activated delayed fluorescence substance, not only efficient generation of a singlet excited state from a triplet excited state but also efficient emission from a singlet excited state, that is, high fluorescence quantum yield is important in a thermally activated delayed fluorescence substance. It is, however, difficult to design a light-emitting material that meets these two.
Patent Document 2 discloses a method: in a light-emitting element containing a thermally activated delayed fluorescence substance and a fluorescent compound, singlet excited energy of the thermally activated delayed fluorescence substance is transferred to the fluorescent compound and light emission is obtained from the fluorescent compound.